1. Field of the Art
This invention relates to an apparatus for controlling a PWM inverter used as an uninterruptible power supply etc.
2. Prior Art
PWM inverters which output constant voltage and constant frequency a.c. voltage are generally used as an uninterruptible power supply used when a failure in the system occurs, or the like. Inverters of this kind input d.c. power to convert it to an a.c. voltage having a predetermined voltage and a predetermined frequency to output such an a.c. voltage. This output a.c. voltage is ordinarily delivered to the load through the interconnecting transformer and the filter. The filter is composed of an a.c. filter reactor and an a.c. filter capacitor. On the input side of the inverter, a d.c. power source capable of having its output voltage adjusted is ordinarily provided. An adjustment of the output voltage of the inverter is made through the voltage adjustment of the d.c. power source. In that case, the inverter shares the role of the frequency adjustment and is controlled by a fixed gate signal corresponding to a predetermined phase reference so that the output voltage becomes equal to a sinusoidal waveform as far as possible.
The filter on the output side of the inverter reduces harmonic components due to switching of power semiconductor elements constituting the inverter, and also serves to prevent overcurrent at the time of short-circuit failure on the side of the load.
For the power semiconductor elements used in the inverter, there are many instances where transistors are used in the small or medium capacity inverter, and gate turn-off thyristors (GTO) are used in the large capacity inverter over several hundred KVA. In the case of GTO used in the large capacity inverter, it is impossible to operate it at a switching frequency as high as that of the transistor. In the case of an output frequency of 50 or 60 Hz, there are many instances where the number of switchings per cycle is less than 10. The number of switchings is limited in view of the switching speed and the loss of the element itself. The harmonics in the inverter output voltage are reduced by the filter provided on the side of the a.c. output of the inverter. Thus, an a.c. voltage similar to the sinusoidal wave is obtained from the output side of the filter. The a.c. voltage thus obtained is delivered to the load.
When a d.c. load is connected through a rectifier on the output side of the filter, many harmonic components except for the fundamental sinusoidal wave are included in the a.c. output current of the inverter. Accordingly, harmonic components are also produced in the a.c. voltage depending on the magnitude of the load power. Such harmonic components can be reduced to some extent by the filter on the a.c. side, but voltage distortion may increase when the load power is large. Since the inverter cannot make such a corrective operation to reduce harmonics due to distorted power on the load side of the inverter in the case of carrying out operation when the number of pulses and the switching phase are fixed, when the load power becomes large, the voltage distortion will also become large. An overvoltage or undervoltage is momentarily produced on the side of the load, resulting in the possibility that equipment forming the load conducts an erroneous operation. In addition, in the case where another load or loads are connected differently from the rectifier load, an adverse influence or effect may be exerted on such load.